The present invention relates to a process for treating liquid manure in order to obtain liquid and solid residues that can be used directly and/or processed by a simple method to prepare useful by-products for various applications.
Liquid manure is known to be the liquid waste obtained on farms practicing intensive animal husbandry. Some of its constituents are feces and urine, which vary with the physiological characteristics of the animals in question, with the feeding regime used, and with the type of animal husbandry practiced. These excrements undergo a series of chemical transformations, owing to microbial fermentation processes, involving hydrolysis. Liquid manure also contains some lost cleaning water, lost drinking water, water lost from the feed, and water used for the cleaning and disinfection of the animal houses. The result of this extra water and the chemical transformations is a product that varies with the characteristics of the method of collecting, extracting and storing this effluent. In view of its quantities, liquid manure causes major environmental contamination, and it is particularly difficult to handle and deal with, because its dry-matter content varies from 1% to 7% in the case of liquid pig manure and from 10% to 20% in the case of liquid cattle manure. Liquid manure is therefore basically a foul-smelling liquid with a low dry-matter content but with a large amount of suspended particles in the colloidal state. These particles come from organic plant nutrients, trace elements and carbon-containing reduced materials. It is therefore important to treat liquid manure in order to eliminate these substances from it and, even more, to convert it into usable materials or by-products.
Apart from its direct application to the fields, liquid manure is traditionally subjected to various means of handling and treatment. Almost all these are based on separation processes, using various mechanical, physical, chemical and mixed methods. However, these separation processes have proved to have a low efficiency of about 5-15%. In addition, one has to bear the cost of the inorganic reagents needed for the chemical coagulation and flocculation. Furthermore, the large amount of additives needed is an environmental disadvantage. Various processes are already known for the treatment of liquid manure. These generally start with the separation of the liquid and solid phases, after which these two phases are treated differently, according to the process employed.
In some cases the separate phases are subjected to a thermal treatment to obtain a dry or half-dry residue, and Spanish Patent No. P 9600968 can be cited in this connection. However, this is an expensive process, because of the large amount of heat required, apart from which its environmental rating is doubtful.
In other cases, the liquid phase is subjected to biological purification, and European Patent EP 0558421 and Spanish Patents Nos. 9402528, 9601316 and 9702540 can be cited in this connection. However, these processes call for the prior treatment of the solid and the liquid phase, are slow, and require large and expensive equipment. In addition, they are very sensitive to variations in the charge (due to the variability of liquid manure), to the high toxicity of the trace elements in it and to the toxicity for the microorganisms involved in the biological process, because of its antibiotic content.
Finally, there are some known processes for the treatment of liquid manure that use precipitation with an electrically charged flocculent, as described in European Patents EP 0508023 and EP 0508024. However, the precipitation of liquid manure with flocculants by the methods described in these patents is not as effective as it should be, because the addition of the flocculant is not carried out in a situation that is the most suitable for the liquid manure, and furthermore the result of the process varies with the type of reagent used. It should be stressed that the use of flocculants generally does not guarantee precipitation, because no account is taken here of the variability of liquid manure and especially its total suspended solid (TSS) content andxe2x80x94within thisxe2x80x94its volatile suspended solid (VSS) content, or of the relationship between these and the plant nutrients (nitrogen, phosphorus and potassium), the trace elements (zinc, copper and chromium), and the remaining mass (in terms of total organic carbon or TOC), as well as the chemical oxygen demand (COD) and the biological oxygen demand (BOD), which are almost entirely due to colloidal and dissolved matter. One of the reasons for this is that chemical precipitation of colloidal and/or dissolved substances is extremely difficult.
The aim of the present invention is to provide a process for the treatment of liquid manure by homogenizing the liquid manure and by the ionic transfer of electrons using organic polymers (polyacrylamide) according to the physicochemical characteristics of the colloids contained in the homogenized liquid manure to be treated. Owing to the use of a specific method, the controlled mixing of the polymers and the liquid manure brings about the formation of soluble molecules activating the copolymers, with the activation of long molecular chains, which destabilize the charged particles in the suspension, absorb them and form molecular bridges between the various suspended particles. The final result is that particle agglomerates are formed, giving large and firm floccules. This new physical form enables the resulting solids to be treated efficiently and separated from the liquid phase by a special method of removal. This gives final liquid and solid fractions that can be used either directly or after the application of a simple treatment or well-known customary operations. Treatment with polyacrylamide is not only highly effective for the agglomeration of colloids and the solids in suspension and for separating the organic nutrients from the liquid manure but it also needs only a very small amount of additives (polymers and copolymers), which is advantageous and sustainable from the environmental point of view, which is a requirement.